Cold rolled iron chromium alloy article



Patented Nov. 13, 1 934 Harry S. George, Massapequa, N. Y., assignor to Elcctro Metallurgical of West Virginia Company, a corporation No Drawing. Application December 21, 1928, Serial No. 327,750

8 Claims.

My invention relates to iron-chromium alloy articles containing from 8% to 40% chromium and having a cold rolled finish and a high ductility.

Steel which has been given a cold rolled finish is desired on account of its characteristic bright, smooth and compact surface. It is possible to make ordinary carbon steel and many alloy steels with this kind of a surface finish because the 10 action of the surfacing rolls on the annealed material does not destroy useful characteristics of the steel and because a desired stiffness is imparted to the steel in cold rolling. When ironchromium alloys containing more than about 5 8% of chromium, such as stainless steel or rustless iron are given a cold rolling treatment like that given to the carbon steels, the ductility of the material is practically destroyed or is so greatly reduced that the articles have little or no practical utility.

The resistance of these alloys to corrosion depends to some extent upon the physical structure of the alloys. Cold rolling changes the physical structure of the alloys and lowers their resistance to corrosion. The ductility and resistance to corrosion may be restored by annealing but heretofore the characteristics of a cold roller surface could not be preserved because a thin oxide film was formed which could not be removed without destroying the bright, compact and'smooth surface texture.

It is among the objects of my invention to provide ductile iron-chromium alloy articles having a cold rolled surface finish and'to provide such articles free from surface oxides.

The best cold rolled surfaces are produced if the annealed articles are cold rolled when they are free from oxide. I take the annealed alloy article from which the oxide has been removed 40 by any desired method and give it a'cold rolled finish. The, rolling reduces its ductility to an undesirable degree. It is then given a heat treatment sufficient to restore the ductility. The treatment may consist of heating to about600 to 800 C. A short heating such as a minute at this temperature is usually sufiicient to restore the ductility to the cold rolled alloy but a longer heating or a higher temperature may be used in some cases without unduly oxidizing the surface or destroying the finish. However, I prefer to heat the metal at such temperatures and for as short periods of time as will suffice to restore the desired ductility and to form an oxide which can 1 to be described.

For most purposes, box annealing is used to restore ductility in order to affect the surface as little as possible and to producea surface oxide of minimum thickness. '1 have obtained good results in box annealing by heating in a reducing or a non-oxidizing atmosphere which is free from hydrocarbon vapors, or which is at least not strongly oxidizing. The preferred method of box annealing consists in placing a few lumps of wood charcoal in the annealing box but of contact with the metal, heating to about 600 C. and holding the metal at this temperature for about five minutes. A thin oxide is formed on the metal which is practically immune. to pickling acids.

To clean the oxide from the article, I use the method described in my co-pending application entitled Method of removing oxide coating from metals, Serial No. 327,756, filed Dec. 21, 1928. The method consists of treating the metal in an alkali bath which preferably contains an oxidizing agent. The surface oxide is then removed in an acid bath. For the present purposes of illustration, satisfactory results are obtained by using 20% sodium hydoxide solution at about C. in which chlorine has been bubbled until a cloud of oxygen appearsinthe solutionbutmore or less concentrated alkali solutions containing from 15% to 30% sodium hydroxide may be used. Any acid pickling bath may be used which does not attack the metal and destroy the cold rolled finish. A dilute solution of nitric acid makes a satisfactory bath when it is heated to about 70 C. Small additions of hydrochloric acid may be used to advantage withthis bath without affecting the surface finish. A bath containing about 3% nitric acid generally gives the best results but less concentrated acid baths may be used. It is generally advantageous to have not more than 1% hydrochloric acid present when mixed acid is. used. For cleaning iron-chromium alloys containing 8% to 20% chromium the concentration of the mixed nitric and hydrochloric acids may be from 1% to 10%. For cleaning rustless iron.containing 16% to 18% chromium the best results are obtained by using a 3% nitric acid bath containing less than 1% hydrochloric acid but with alloys of high chromium content more hydrochloric acid may be used. I

The following table shows the tensile strength and ductility of three rustless iron articles con- .65

taining 0.07% carbon and 18% chromium A) after hot working and box annealing at 800 to 950 C. (B) after pickling and cold roll ng and C) after the cold rolled finish was given to a clean annealed metal article and it was then given a box annealing and cleaning as hereinbefore described.

Ultimate tensile Percent 5 strength elongation pounds per in 2 inches square inch A. Box-annealed and not cold-rolled.'...... 75,000 16 B. Gold-rolled 110, 000 l C. Gold-rolled, heat treated and cleaned-- 78, 000 17 Comparison of examples A and B shows that cold rolling practically destroyed the ductility of the article. A comparison of examples A and'C shows that the finished article produced in accordance with the foregoing description had ductility throughout about the same as a hot worked annealed article of the same composition.

Various modifications of my invention may be made such as removing all the oxide after the article is annealed. Therefore, I desire to be limited only by the prior art and by the invention as.

defined inthe annexed claims.

I claim as my invention: 7 1. A ductile article consisting of an ironchromium alloy. containing about 8% to. chromium having a cold-rolled surface finish and consisting throughout of metal in an annealed condition. 7 Y

2. A ductile article consisting of an iron-chromium alloy containing about 8% to 20% chro- I mium having a cold-rolled surface finish free from oxide, said article being free from the surrness imparted by cold rolling.

3; An iron-chromium alloy articlecomposed mainly of iron and containing about 8% to 40% of chromium having a cold-rolled surface finish,

said article having ductility throughout about the same as a? hot-rolled box annealed article of the same composition.

4. An iron-chromium alloy article composed mainly of iron and containing about 8% to 40% a of chromium having a cold-rolled finish free from oxide and a ductility throughout about the same as a hot-rolled box annealed article of the same composition. 5. A ductile article consisting of an iron-chromium alloy containing less than .1% of carbon and about 8% to 40% chromium having a coldtoo a cold-rolled surface finish, said article having ductility throughout about the same as a hotrolled box annealed article of the same composition.

8. An iron-chromium alloy article composed mainly of iron and containing about 8% to 40% 1' chromium and less than .1% of carbon having a cold-rolled finish free from oxide and a ductility throughout about the same as a hot-rolled boxannealed article or the same composition.

' HARRY S; GEORGE. 

